Solid oxide fuel cell (SOFC) generators comprise fuel cells which are electro-mechanically connected together. The individual solid oxide fuel cells are high temperature, electro-chemical devices fabricated primarily from oxide ceramics. Each fuel cell typically contains an oxygen ion conducting solid electrolyte, such as yttria stabilized zirconia. The electrolyte is usually a thin dense film which separates two porous electrodes comprising a cathode and an anode. The cathode, which is maintained in an oxidizing atmosphere during operation of the fuel cell, is usually an oxide that is doped to obtain high electrical conductivity, such as lanthanum manganite doped with strontium. The anode, which is maintained in a reducing atmosphere during operation, is usually a cement such as nickel-zirconia. Each fuel cell usually includes an interconnection comprising a dense, electrically conductive oxide material which is stable in both reducing and oxidizing environments, such as doped lanthanum chromite. The interconnections permit the anodes and cathodes of adjacent cells to be electrically connected in series. These connections are typically made using an electrically conductive, compliant material such as nickel felt.
Prior art solid oxide fuel connections are disclosed in U.S. Pat. Nos. 4,431,715, to Isenberg, 4,490,444 to Isenberg, 4,748,091, to Isenberg, 4,791,035 to Reichner, 4,833,045, to Pollack et al., 4,876,163 to Reichner, 5,258,240 to Di Croce et al., 5,273,828, to Draper et al., each of which are incorporated by reference herein.
Conventional SOFC generator designs for units on the order of 20 or 25 kWe have included multiple fuel cells grouped into bundles, with bundle-to-bundle connections made by in-situ sintering of nickel felts. The bundle-to-bundle connection is accomplished in the early stages of actual operation when external pressure, provided by compressed insulation material, forces the individual pre-sintered cell bundles together to permit high-temperature bundle-to-bundle felt sintering. This method of bundle-to-bundle attachment occurs within cell bundle rows and also between cell bundle rows by row connect assemblies to form a serpentine layout of the generators. However, due to the increased size and overall layout of larger SOFC generators on the order of 100 kWe, transferring of force from the periphery to the central regions of the generator cell stack is more difficult. The conventional in-situ bundle-to-bundle connection method therefore makes it difficult to assure consistent and adequate felt contact pressure and cell positioning for larger cell stacks.
An alternative to in-situ sintering is to pre-sinter the cell bundle rows in a large, high-temperature furnace at temperatures of about 1050.degree. C. However, due to the large size of the generator stacks, even furnace sintering proves to be very difficult in producing reliable felt bonding and cell positioning.
An additional drawback associated with conventional cell bundle sintered connections is the difficulty of cell removal and replacement. Suitable techniques have not yet been developed to practically or economically remove and replace cells for generator maintenance or repair.
The present invention has been developed in view of the foregoing and other deficiencies of the prior art.